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The anterior cruciate ligament (ACL) consists of two major fiber bundles, namely the anteromedial and posterolateral bundle. When the knee is extended, the posterolateral bundle (PL) is tight and the anteromedial (AM) bundle is moderately lax. As the knee is flexed, the femoral attachment of the ACL becomes a more horizontal orientation; causing the AM bundle to tighten and the PL bundle to relax. There is some degree of variability for the femoral origin of the anterome-dial and posterolateral bundle. The anteromedial bundle is located proximal and anterior in the femoral ACL origin (high and deep in the notch when the knee is flexed at 90 degrees ); the posterolateral bundle starts in the distal and posterior aspect of the femoral ACL origin (shallow and low when the knee is flexed at 90 degrees ). In the frontal plane the anteromedial bundle origin is in the 10:30 clock position and the postero-lateral bundle origin in the 9:30 clock position. At the tibial insertion the ACL fans out to form the foot region. The anteromedial bundle insertion is in the anterior part of the tibial ACL footprint, the posterolateral bundle in the posterior part. While the anteromedial bundle is the primary restraint against anterior tibial translation, the posterolateral bundle tends to stabilize the knee near full extension, particularly against rotatory loads.
This work studied the fibre bundle anatomy of the anterior cruciate ligament. Three functional bundles--anteromedial, intermediate, and posterolateral--were identified in cadaver knees. Their contributions to resisting anterior subluxation in flexion and extension were found by repeated tests after sequential bundle transection. Changes of length in flexion and extension and in tibial rotation were measured. None of the fibres were isometric. The posterolateral bundle was stretched in extension and the anteromedial in flexion, which correlated with increased contributions to knee stability and the likelihood of partial ruptures in these positions. Tibial rotation had no significant effect. The fibre length changes suggested that the 'isometric point' aimed at by some ligament replacements lay anterior and superior to the femoral origin of the intermediate fibre bundle and towards the roof of the intercondylar notch.
In OS X, shared resources are packaged using standard frameworks and umbrella frameworks. Both types of framework feature the same basic structure and can contain resources such as a shared library, nib files, image files, strings files, information property lists, documentation, header files, and so on. Umbrella frameworks add minor refinements to the standard framework structure, such as the ability to encompass other frameworks.
Frameworks are packaged in a bundle structure. The framework bundle directory ends with the .framework extension, and unlike most other bundle types, a framework bundle is presented to the user as a directory and not as a file. This openness makes it easy for developers to browse any header files and documentation included with the framework.
Framework bundles use a bundle structure different from the bundle structure used by applications. The structure for frameworks is based on an earlier bundle format, and allows for multiple versions of the framework code and header files to be stored inside the bundle. This type of bundle is known as a versioned bundle. Supporting multiple versions of a framework allows older applications to continue running even as the framework binary continues to evolve.
When you build a new framework project in Xcode, the build environment creates a versioned bundle structure for you automatically. Listing 1 shows the basic directory structure of the resulting bundle.
In this listing, the Versions directory is the only real directory at the top level of the bundle. Both MyFramework and Resources are symbolic links to items in Versions/A. The reason for the symbolic links is that directory Versions/A contains the actual contents of the framework. It contains both the executable and the resources used by the framework.
Important: For the linker to find and link the dynamic library, the name of the framework (without the .framework extension), the symbolic link, and the dynamic library must be the same.
Frameworks typically include more directories than just the Resources directory. For example, Headers, Documentation, and Libraries. Thus, adding a Headers directory to the example in Listing 2 would result in a framework like the one shown in Listing 3.
To create additional directories in your framework, you must add build phases to the appropriate target in Xcode. The Copy Files build phase lets you create directories and copy selected files into those directories. Table 1 lists some of the typical directories you might add to your framework.
Contains HTML or PDF files describing the framework interfaces. Typically, documentation directories do not reside at the top level of your framework. Instead, they reside inside your language-specific resource directories.
Frameworks require the same sort of configuration as any other type of bundle. In the information property list for your framework, you should include the keys listed in Table 2. Most of these keys are included automatically when you set up the framework properties in Xcode, but you must add some manually.
For more information on configuration and information property lists in general, see Runtime Configuration Guidelines. For specific details about each of the keys in Table 2, read Information Property List Key Reference.
The structure of an umbrella framework is similar to that of a standard framework, and applications do not distinguish between umbrella frameworks and standard frameworks when linking to them. However, two factors distinguish umbrella frameworks from other frameworks. The first is the manner in which they include header files. The second is the fact that they encapsulate subframeworks.
The purpose of an umbrella framework is to provide all the necessary interfaces for programming in a particular application environment. Umbrella frameworks hide the complex cross-dependencies among the many different pieces of system software. Thus you do not need to know what set of frameworks and libraries you must import to accomplish a particular task. Umbrella frameworks also make faster builds possible through the use of precompiled headers.
An umbrella framework simply includes and links with constituent subframeworks and other public frameworks. An umbrella framework encompasses all the technologies and APIs that define an application environment or a layer of system software. It also provides a layer of abstraction between what outside developers link their programs with and what Apple engineering provides as implementation.
A subframework is structurally a public framework that packages a specific Apple technology, such as Apple events, Quartz, or Open Transport. However, a subframework is public with restrictions. Although the APIs of subframeworks are public, Apple has put mechanisms in place to prevent developers from linking directly with subframeworks (see Restrictions on Subframework Linking). A subframework always resides in an umbrella framework installed in /System/Library/Frameworks, and within this umbrella framework, its header files are exposed.
Some umbrella frameworks include other umbrella frameworks; this is particularly the case with the umbrella frameworks for the Carbon and Cocoa application environments. For example, both Carbon and Cocoa (directly or indirectly) import and link with the Core Services umbrella framework (CoreServices.framework). This umbrella framework, in turn, imports and links with subframeworks such as Core Foundation.
The exact composition of the subframeworks within an umbrella framework is an internal implementation detail subject to change. By providing a level of indirection, umbrella frameworks insulate developers from these changes. Apple might restructure the subframeworks within an umbrella framework and might add, rename, or remove the header files within subframeworks. If you include the master header file for the subframework, these changes should not affect your programs.
Physically, umbrella frameworks have a similar structure to standard frameworks. One significant difference is the addition of a Frameworks directory to contain the subframeworks that make up the umbrella framework.
Listing 4 shows a partial listing of the Core Services framework. (The contents of the subframeworks are not included since they are not referenced anyway.) As with standard frameworks, the top-level items are symbolic links to items deeper within the framework directory structure. In this case, the linked libraries and directories are located in folder A of the framework.
Unlike standard frameworks, the Headers directory of an umbrella framework contains a more limited set of header files. It does not contain a collection of the headers in its subframeworks. Instead, it contains only the master header file for the framework. When referring to an umbrella framework in your source files, you should include only the master header file. See Including Frameworks for more information.
Know Yourself engages young minds in understanding anatomy, physiology, and psychology. These books set a foundation for discovering how the body functions, building confidence, and making healthy decisions for yourself. Our pedagogical approach includes a variety of learning modalities, spiral curriculum design, and culturally responsive and representative content.
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